In recent years, there has been developed a fiber laser using a semiconductor laser as a pump light source and a rear-earth-doped fiber as an amplifying medium. For example, a high-power semiconductor laser diode (LD) formed of a material based on gallium arsenide (GaAs) has been used as a pump light source of such a fiber laser. The power of fiber lasers has been increased in recent years. Accordingly, it has also been strongly demanded to increase the power of laser diodes. For example, the output of over 10 W has been required for one laser diode chip, or the output of almost 20 W has been required in some cases.
As the power of laser diodes is increased, the amount of heat generated from laser diodes also increases. The characteristics of the laser diode is also problematically deteriorated by heat generated in a laser diode itself. For example, assuming that the photoelectric conversion efficiency of a laser diode that has an output of 15 W is 60%, the laser diode generates the amount of heat as large as 10 W.
Generally, a laser diode is mounted on a plate, which is referred to as a submount, and used in the form of a module in which the laser diode is integrated with the submount. Therefore, a submount that has a high heat conductivity and exhibits high heat dissipation performance is required to avoid deterioration of the characteristics of a laser diode due to heat generated by the aforementioned increase of the power of the laser diode.
Furthermore, if a thermal expansion coefficient of a laser diode is greatly different from a thermal expansion coefficient of a submount, a stress may be generated by the thermal hysteresis upon mounting the laser diode on the submount, resulting in deteriorated characteristics of the laser diode. Accordingly, a submount is required to be formed of a material having a thermal expansion coefficient that is close to a thermal expansion coefficient of a laser diode.
Generally, a laser diode has a bottom surface having an electrode formed thereon, which is connected to a submount. Thus, an electric current is supplied to the laser diode through the submount. Therefore, in order to obtain a high power with high efficiency, reduction of an electric resistance of the submount is required in addition to enhancement of the heat dissipation performance of the submount.
Thus, because of an increased power of a laser diode, a submount on which the laser diode is mounted requires 1) enhanced heat dissipation performance, 2) a thermal expansion coefficient that is close to that of the laser diode, and 3) a reduced electric resistance.
However, it is difficult to find out a material that simultaneously meets the aforementioned three requirements. For example, when a material that generally exhibits high heat dissipation performance, such as ceramics, is used as a material for a submount, high heat dissipation performance can be achieved. Nevertheless, since ceramics is an electrical insulator, the electric resistance of the submount becomes very high.
Patent Literature 1 discloses that silicon is used as a material for a submount (see, e.g., paragraph [0024]). Thus, the electric resistance of the submount can be reduced by use of silicon. However, silicon has a heat conductivity of 150 W/mK, which is not so high. Accordingly, use of silicon cannot achieve high heat dissipation performance.
Furthermore, Patent Literature 2 discloses an embodiment using CuW as a material for a submount disposed right below a laser chip (see paragraphs [0028] and [0029] and FIG. 7). CuW has an electric resistivity as low as 10−8 Ωm and a heat conductivity of 170 W/mK, which is higher than that of silicon. Additionally, CuW has a thermal expansion coefficient of about 6.5 ppm/K, which is close to a thermal expansion coefficient of gallium arsenide (about 5.9 ppm/K), which may also be used as a material for a semiconductor laser diode. Accordingly, CuW has widely been used as a material of a submount for a high-power semiconductor laser. However, any material that exhibits higher heat dissipation performance than CuW is needed to output a power that is higher than 20 W.
Patent Literature 2 discloses embodiments stacking a diamond submount, a silicon carbide (SiC) submount, or the like in order to improve heat dissipation performance of the submount. However, those submounts have a high electric resistivity (e.g., 10−4 Ωm). Therefore, if an electric current is supplied to a laser diode through those submounts, that an optical output cannot efficiently be obtained because the electric resistance of the entire laser module is increased.
Furthermore, Patent Literature 3 discloses embodiments using vapor-deposited diamond or cBN (cubic boron nitride) as a material for a submount (see paragraphs [0055]-[0057] and FIG. 7). However, both of vapor-deposited diamond and cBN have a high electric resistivity. With a submount made of such a material, the electric resistance of the entire laser module is increased as with the submounts of Patent Literature 2, so that an optical output cannot efficiently abe obtained.